Austenitic stainless steels, for example, such as SUS 304, SUS 316, SUS 310S and others, which are regulated in JIS G 4304 (2005), are materials containing Cr and Ni as main alloying elements, and they are excellent in corrosion resistance.
However, in the present state of affairs, restrictions are imposed on the contents of the respective constituent elements for any of the austenitic stainless steels. In particular, the content of P, which is an impurity element, is generally limited to a level not more than 0.045%. Furthermore, the P content in the welding materials to be used for welding these steels, for example in “austenitic stainless steel wire rods for welding” described in JIS G 4316 (1991), is limited to a further lower level, namely not more than 0.030%.
On the other hand, it is known that when P, which is essentially an impurity element is contained at a high concentration, it contributes toward precipitation hardening by making M23C6 carbides finer, hence leads to improvements in creep strength.
Therefore, for example, the Patent Documents 1 to 3 propose techniques in which P is allowed to be contained in high concentrations.
That is to say, the Patent Document 1 discloses “an improved heat resisting steel for valves”. The said steel contains 0.05 to 0.40% of P which produces a precipitation hardening-promoting effect, and therefore the high temperature-strength thereof is markedly increased.
The Patent Document 2 discloses “an austenitic stainless steel excellent in creep strength” which is added 0.03 to 0.08% of P to a very low carbon austenitic stainless steel having a C content of not higher than 0.01%.
Further, the Patent Document 3 discloses “an austenitic stainless steel” which contains 0.05 to 0.30% of P and is excellent in high temperature strength.
Thus, when P is contained at high concentrations, it contributes toward rendering M23C6 carbides finer and can increase the creep strength.
However, the increase of the P content in those austenitic stainless steels, in particular which show fully austenitic solidification such as SUS 310S, causes a deterioration of weldability. That is to say, the incidence of cracking, which occurs when the strain resulting from the solidification shrinkage or thermal shrinkage exceeds the deformability of the weld metal, in particular, in the stage which is close to the end of the weld solidification process in which a filmy liquid phase is present mainly along the crystal grain boundaries (hereinafter such cracking is referred to as “weld solidification cracking”), is increased.
Therefore, in particular, in the case of austenitic stainless steels which show the fully austenitic solidification, restrictions are imposed on the increased P contents from the weldability viewpoint, for example in the Non-Patent Document 1, it is shown that the P content should be rigidly restricted.
The technique disclosed in the Patent Document 1 is indeed suited for such fields of application as exhaust valves and stop valves, but does not take into consideration the marked increase in susceptibility to the weld solidification cracking resulting from the increase of P content. Therefore, it is very difficult to use the steel disclosed therein as a steel product requiring welding; in particular, the steel cannot be used in austenitic stainless steel products which show the fully austenitic solidification.
In the Patent Document 2, it explains that only the upper limit of the P content should be set to 0.08% so that the weldability may not be impaired; there is no description at all of what should be taken into consideration for reducing the susceptibility to the weld solidification cracking in such an austenitic stainless steel which shows the fully austenitic solidification in the stage that the austenite solidifies as a primary crystal. Therefore, the austenitic stainless steel disclosed in the Patent Document 2 cannot be used, in particular, in an austenitic stainless steel product which shows the fully austenitic solidification.
The technique disclosed in the Patent Document 3 can indeed be applied widely in those fields where steel pipes, steel plates, steel bars, steel castings, steel forgings and the like which are required to have high temperature strength and corrosion resistance. However, no sufficient study has been made of the increased P content-due weld solidification cracking in such austenitic stainless steels which show the fully austenitic solidification. Therefore, when the austenitic stainless steel disclosed in the Patent Document 3 is used as an austenitic stainless steel product which shows the fully austenitic solidification, it is not always possible to ensure excellent resistance to the weld solidification cracking.
Patent Document 1: Japanese Examined Patent Publication No. 37-17113
Patent Document 2: Japanese Unexamined Patent Publication No. 62-267454
Patent Document 3: International Publication WO 2006/106944
Non-Patent Document 1: Y. Arata, F. Matsuda and S. Katayama: Transactions of JWRI, Vol. 6-1 (1977), pp. 105 to 116